How to transfer data between two children activities in android?

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http://stackoverflow.com – I have 3 Activities: 1 Parent and 2 children. Let's call them simply Activity1, Activity2a and Activity2b; Activity2a and Activity2b are both started from Activity1. How can I transfer data from Activity2a to Activity2b through the parent? Any ideas? I think I have to use intents but don't know how. (HowTos)